This invention relates to a rechargeable electrochemical cell and more particularly to an improved high compression radial seal for retaining electrolyte within the confines of a rechargeable electrochemical cell.
Many state of the art rechargeable electrochemical cells, such as nickel-cadmium cells, are comprised of at least two opposite polarity electrode plates loaded with an electrochemically active material. The electrodes are separated by a strip of separator and the assembly is spirally wound into a cylindrical container or can. Electrolyte is introduced into the can and is retained in the pores of the electrodes and the separator. The cell includes a cover which cooperates with the container to provide a sealed environment within the cell wherein the various electrochemical reactions may occur during operation of the cell. It is necessary to introduce an insulating material between the cover and the container wall which also acts as the sealing material to prevent leakage of electrolyte from the cell at the interface between the cover and the container.
It is important that the seal associated with the above-described electrochemical cell maintain an effective sealing function throughout the life of the cell. If the sealing function is not maintained, a number of undesireable conditions adverse to effective cell operation may result. First, water from the electrolyte can be lost from the cell through evaporation or means causing the cell to no longer function as an effective means of storing and delivering electrical energy. Secondly, electrolyte leaking from the cell may, due to its corrosive nature, contaminate and damage exterior components of the cell or, more importantly, elements of the device in which the cell is installed.
Radial seal mechanisms heretofore known and used to prevent leakage of electrolyte from electrochemical cells have proved not entirely suitable for that purpose. More specifically, the material used in the seal itself has usually been comprised of a crystalline polymer such as nylon. These crystalline polymer materials may be deformed or compressed only up to about 20% to 25% of their free radial thickness without suffering a reduction in sealing capability. Furthermore, over this range of deformation the maximum compressive stress attainable before failure of the material has not proved to be sufficient to provide effective sealing for a sustained period over the life of the electrochemical cell.
Deformability and high compressive stress capability are both important characteristics in a radial seal material. The material must be capable of withstanding the high radial compressive forces necessary to achieve a seal impervious to the passage of electrolyte. It has been discovered that the leakage of electrolyte, such as potassium hydroxide associated with a nickel-cadmium electrochemical cell, past a radial seal may only be eliminated by seals under high radial compressive force hereto unapplied in radial seals in the prior art.
Furthermore, the material must have the capability to withstand high compressive stress in order to accommodate high radial compressive forces which may occasionally result when various cell component parts are manufactured to dimensions at the outer limits of the normal tolerance associated with the component parts. The material also must be capable of substantial radial deformation or compression from its free radial thickness in order to be compatible with the aforementioned tolerance "stack-up" encountered with the manufacture of electrochemical cells in large quantities and at high rates.
As is well known, the individual component parts of an electrochemical cell, as with many products, each have tolerances associated with critical dimensions or specifications of the parts. The smaller the tolerances the greater the cost of the component and the greater the cost and selling price of the cell. It has been found that with tolerances necessary to produce the cell at a competitive cost for sale at a competitive price, a cell exiting a production line may be assembled with parts which have dimensions at the outer limits of the tolerance band. More specifically, with the parts intended to provide the radial seal, the actual dimensions of the parts may be such that the gap in which the seal resides may be at a maximum while the amount of seal material may be at a minimum. Accordingly, the amount of compression of the seal member will be less than intended. Cells so made may exhibit little or in some cases no sealing integrity. As a remedy to this problem, the component part may be redimensioned so that the amount of seal material present under minimal tolerances is increased. This remedy, however, can only be appropriate where the material exhibits a proper stress/strain relationship. Otherwise the material will be overstressed when the tolerance stack up is reversed; that is to say, the gap is at a minimum and the amount of seal material is at a maximum. An appropriate stress/strain relationship would be found in a material capable of high compressive stress over a substantial range of deformation.
In order to achieve the high radial sealing forces necessary to effect a seal of high integrity and in order to be compatible with tolerance stack up, the radial seal material should be capable of undertaking a radial compressive stress of up to about 40,000 psi and a radial deformation or compression from its free height of up to 50%. Prior art crystalline polymer materials such as nylon are not capable either of this radial compressive stress or this radial deformation or compression without compromise of sealing integrity. The invention disclosed hereinafter overcomes this shortcoming of prior art crystalline polymer materials.
In addition to the seal itself, other cell component parts cooperate in the sealing function of a radial seal of an electrochemical cell; namely, the container and the cover assembly. As higher compressive sealing forces are introduced in the high volume manufacture of electrochemical cells having radial seals, the tendency of the container to spring back is increased. More specifically, a radial seal function is achieved by deforming the cell container wall radially inwardly past its elastic limit to thereby compress the radial seal material between the container wall and the seal cover assembly. When the deforming force is removed from the container wall, the wall has a tendency to move radially outward or spring back in response to the compressive force present in the seal material. The more the wall is deformed the higher the compressive force generated in the seal and the greater the springback of the container wall after removal of the deforming force. One prior art approach in limiting the amount of springback of the container wall is disclosed in U.S. Pat. Nos. 3,062,910 and 3,185,595. These patents teach springback reduction by providing a metal reinforcing ring at one end of the cell container adjacent that part of the wall to which the deforming force is applied. The metal reinforcing ring is deformed along with the container wall and the reduction in springback is accomplished due to the increase in radial cross-section of deformed material added by the presence of the reinforcing ring. However, this approach is disadvantageous at least because of the added cost of fabrication, material and handling associated with the additional separate component part in the form of a reinforcing ring. In addition, the use of a reinforcing ring may not provide sufficient retaining force to prevent the cell top from bursting at high internal pressures, generally above 1,200 psi. The invention hereinafter disclosed addresses these shortcomings associated with a reinforcing ring.
While some prior art electrochemical cells have utilized amorphous polymers such as polysulfone in radial seals along with reinforcing rings, such cells also suffer from the aforementioned disadvantages associated with reinforcing rings. While other prior art electrochemical cells have also employed amorphous polymers as seals in axial seals these cells suffer from the disadvantages usually associated with axial seal structures; namely, low and erratic sealing forces under high volume production conditions and excessive springback. Accordingly then, the prior art does not admit of a radial seal employing an amorphous polymer in combination with other sealing structures whereby a sealing function of high integrity is produced capable of withstanding high deformation and effecting high compressive sealing forces and capable of also preventing cell top bursting at high internal pressures.
Therefore, it is an object of the present invention to provide a new improved electrochemical cell configuration having a radial seal of high sealing integrity.
It is another object of the present invention to provide a radial seal capable of withstanding high radial compressive forces.
It is still another object of the present invention to provide a radial seal capable of withstanding a high degree of deformation without any compromise of sealing integrity.
It is yet another object of the present invention to provide a radial seal which is compatible with tolerance limits normally associated with component parts of an electrochemical cell.
It is still another object of the present invention to provide a radial seal for an electrochemical cell in which the springback of the cell container wall is reduced or eliminated and in which the cell has a high capability to withstand high burst pressures.
It is yet another object of the present invention to provide a method of making an electrochemical cell having a radial seal of high integrity.